Ultrasonic imaging apparatus

ABSTRACT

Ultrasonic imaging apparatus including ultrasonic transducers for emitting acoustic wave beams of various apertures, apparatus for directing two acoustic wave beams of substantially different focussing distances on the same beam direction, circuitry for receiving the reflected waves corresponding to the beam directions, apparatus for modulating the scanning lines corresponding to the beam directions of the reflected acoustic waves, and apparatus for composing the scanning lines corresponding to the two acoustic beams directed on the same beam direction into a single scanning line for display.

This is a continuation of application Ser. No. 06/238,850, filed Feb.27, 1981, now abandoned.

BACKGROUND OF THE INVENTION

This invention relates to ultrasonic imaging apparatus and particularlyto ultrasonic diagnosis apparatus for forming tomographic images ofinternal elements of a living body.

In ultrasonic diagnosis apparatus, acoustic beams are emitted into asubject, echoes corresponding to the direction of the beam positions arereceived, scanning lines corresponding to the beam direction with theechoes are modulated to conform them to a display apparatus, and atomographic image of the subject is obtained by using the acoustic beamsof a plurality of beam directions.

For example, ultrasonic diagnosis apparatus techniques for causinglateral resolution to become more precise include the following:

(i) Apertures of emitted acoustic beams are caused to changecorresponding to the required depth being diagnosed, that is, in casethe required depth is relatively shallow the beam aperture is caused tobe small to improve the lateral resolution. In case the required depthis deep, the beam aperture is caused to be relatively large to providethe scope of the long distance acoustic field, which narrows as itpenetrates, and checking deterioration of the lateral resolution becauseof the extensions of the beams in the long distance acoustic field.Hence, the smaller the beam aperture is, the shallower the scope of theshort distance acoustic field is and the larger the beam aperture is,the deeper the scope of the short distance acoustic field is;

(ii) Focusing the emitted acoustic beams causes the lateral resolutionto become better defined in the vicinity of the required depth beingdiagnosed;

(iii) The received beam aperture in echo-receiving time is caused to besmall in case of the short distance acoustic field and to be large incase of the long distance acoustic field to define the lateralresolution, more precisely, and;

(iv) The received focusing point in echo-receiving time is caused toshift corresponding to the reflective point of the received echo toprovide precise lateral resolution.

These techniques have been mainly realized in a so-called electronicscan type of ultrasonic apparatus and, in the apparatus of theinvention, are put to practical use by properly combining thesetechniques and by adding other techniques to them, as will be described.

An example of conventional concrete techniques for improving the lateralresolution with respect to the direction of the diagnostic depthfollows. In the ordinary ultrasonic diagnosis apparatus, a train ofrepeating rate pulses having identical wave form, for example, as shownin FIG. 1(a), is set up, and a train of acoustic pulses is emitted onthe timing of the rate pulses. An example of echo waves corresponding tothe acoustic pulses is shown in FIG. 1(b). Referring to FIG. 2, thereare carried out the transmission and the reception of the acoustic pulsebeams emitted by, for example, m₁ elements (in the first case twelve) ofunit transducers UT of an electronic scan probe EP. The groupedemissions of the linearly disposed unit transducers are designated T1a,in accordance with the rate pulse RP1a, of FIG. 1(a).

In the first example of FIG. 2, a focus is set at the relatively longdistance point F1a on an imaginary line a1 extending from the probe EPto focus the transmitting and receiving acoustic beams. Thedetermination of the focal point gives the time difference in accordancewith particular patterns as to the timing for driving the unittransducers and as to the timing for processing the received echosignal. As the result, the lateral resolution near the focus F1a (thefocus position is practically coincident with the position of the centerof the beam diameter), namely in the comparatively long distance region,becomes more precise.

Synchronizing the transmission and reception of acoustic pulse beamswith the next rate pulse RP1b is carried out with m₂ elements (in thiscase eight) which are less than m₁ as shown with TIb of the unittransducers UT. In this case, the focus of the transmitting andreceiving beams is set at a point F1b comparatively near the probe EP onan imaginary line a2, set apart a predetermined distance from the linea1. So, the lateral resolution of the image of the portion near thefocus F1b, namely the comparatively short distance region, rises.

Moreover, synchronizing with the next rate pulse RP2a, transmission andreception of the acoustic pulse beams are carried again by m₁ elementsof the unit transducers similar to the TIa, as shown with T2a in FIG. 2.In this case, the acoustic pulse beams focus at a point F2a on animaginary line a3 apart a further predetermined distance from the linea2 and similar to the focus F1a in distance from the probe EP.

Likewise, synchronizing with the following rate pulse RP2b, transmissionand reception of the acoustic pulse beams are carried by m₂ elements ofthe unit transducers UT similar to the T1b as with T2b in FIG. 2. Inthis case, the acoustic pulse beams focus at a point F2b on an imaginaryline a4 set apart another predetermined distance from the line a3 andsimilar to the focus F1b in distance from the probe EP. After this, theabove-mentioned operations are repeated while the driving unittransducer positions and the imaginary line positions are shifted oneafter another as well-known in the art and shown in U.S. Pat. No.4,161,122.

The conventional image display system in an ultrasonic diagnosisapparatus is generally composed as shown in FIG. 3, for example, in caseof a linear scan. A known display apparatus to display echo imagesincludes, for example, a brightness modulation circuit 2 for conductingto a cathode-ray tube of a display apparatus 1 video signals whichinclude the echo data obtained by the transmission and the reception ofthe acoustic beams. A blanking control circuit 3 applies to thebrightness modulation circuit 2 a blanking control signal to blankunnecessary components of the video signals and the returns of thelinear scan in the display apparatus 1. A beam directional sweepingcircuit 4 applies to the display apparatus 1 sweeping waves for thelinear scan about the acoustic beam direction. A scanning line positionsignal generator 5 applies to the display apparatus 1 the scanningposition-designating signals to designate the scanning positioncorresponding to each acoustic beam position in the linear scan.

FIG. 1(c) shows the form of the conventional sweeping wave generatedfrom the beam directional sweeping circuit 4, which sweeping wavedefines a saw-tooth wave synchronized with the rate pulses as shown inFIG. 1(a).

FIG. 1(d), shows the form of the scanning line position-designatingsignal generated from the scanning line position signal generator 5. Thescanning line designating signal defines the stepping wave the level ofwhich varies with every rate pulse.

FIG. 1(e) shows the form of the blanking control signal generated fromthe blanking control circuit 3, the blanking in this case being actuatedwhen the blanking control signal is at low level L, and the unblankingsignal being actuated when the signal is at high level H. As shown, theblanking control signal is in the unblanking state during the secondhalf of the scanning line when the rate pulse RP1a and RP2a isgenerated, i.e. for the long distance region, and the blanking controlsignal is put in the unblanking state at the first half of the scanningline when the rate pulse RP1b or RP2b is generated, i.e., thecomparatively short distance region.

Therefore, the display scanning line of each rate pulse in the imagedisplay, respectively, corresponds to the axis lines a1, a2 . . . of theacoustic beams shown in FIG. 2, and the blanking is actuated as to theshort distance region of the display scanning line when the longdistance focuses F1a, F2a, etc. are set and the blanking is actuated asto the long distance region of the display scanning line when the shortdistance focuses F1b, F2b, etc. are set.

Consequently, the echo images having high lateral resolution in both thelong distance and the short distance are displayed, and echo images ofgood quality are obtained over the extensive scope along the acousticbeam direction, namely, the diagnosis depth direction. Apparatus forobtaining these results is taught in U.S. Pat. No. 4,215,584.

However, the conventional system has shortcomings. The echo imagedisplay according to the above-described system is executed as shown inFIG. 4. As is apparent, in the conventional case, each scanning lineconsists of a displayed region UR and an undisplayed region BR becausethe scanning lines contributing to the display at the short distanceregion are not displayed during display of the long distance region andthe scanning lines contributing to the display at long distance are notdisplayed during display of the short distance region. As a result, thescanning line density is caused to be substantially reduced by a half.In addition, there is also the problem that, due to the discontinuitiesin the scanning lines between the short distance region NE and the longdistance region FE, confusing and undesirable portions are caused at theborder BL between them.

SUMMARY OF THE INVENTION

It is accordingly the object of this invention to provide an ultrasonicimaging apparatus for obtaining an effective pattern of the acousticwave beam according to the diagnosis distance with good lateralresolution without any decrease of scanning line density and withoutdiscontinuities of the scanning lines.

Briefly, this and other objects are achieved in accordance with a firstaspect of the invention, by improving ultrasonic imaging apparatusincluding means for emitting acoustic wave beams of various aperturesinto a subject, means for receiving the reflected waves corresponding tothe beam directions, means for modulating scanning lines correspondingto the reflected beam directions and means for displaying tomographicimages of the subject based on a plurality of the modulated scanninglines, wherein the improvement comprises beam control means fordirecting two acoustic wave beams of substantially different lengths onthe same beam direction and display control means for composing themodulated scanning lines of the echo reflections of the two acousticwave beams into a single scanning line on the displaying means.

The accompanying drawings, which are incorporated and constitute a partof this specification, illustrate one embodiment of the invention andtogether with the description, serve to explain the principles of theinvention.

BRIEF DESCRIPTION OF THE DRAWING

FIGS. 1(a) through (e) are time charts for explaining the operation of aconventional ultrasonic diagnosis apparatus;

FIG. 2 is a schematic diagram of assistance in explaining the operationof a conventional electronic scan probe;

FIG. 3 is a block diagram illustrating the construction of the imagedisplay system in a conventional ultrasonic diagnosis apparatus;

FIG. 4 is a diagram for explaining the display image according toapparatus of FIG. 3;

FIG. 5 is a block diagram illustrating the construction of an embodimentof this invention;

FIG. 6 is a schematic diagram showing portions of the control apparatusof FIG. 5;

FIGS. 7(a) through (f) are, timing charts for explaining the operationof the embodiment of FIG. 5;

FIG. 8 is a resultant pattern of two acoustic beams of according to theembodiment of FIG. 5;

FIG. 9 is a diagram for explaining the display image of the embodimentof FIG. 5; and

FIGS. 10(a) through (f) are time charts for explaining an alternateembodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 5, this embodiment of the invention teaches a systemcombining a dynamic focus method and a dynamic aperture method relatingboth to the transmission of sonic beams and the reception of thereflected echo waves.

A probe 11 is provided with a plurality of unit transducers as shown inFIG. 8, desirably arranged for transmission and reception of acousticwaves and a group of pulsers 12 for causing the unit transducers of theprobe 11 to oscillate. A first group of delay lines 13 gives apredetermined delay time to trigger pulses generated by pulse oscillator14 for driving the group of pulsers 12. The delay lines 13 are also usedto focus the transmission of the sonic beams as known in the art.

Echo waves reflected from a subject under examination are received andconverted into electrical echo signals by the probe 11.

The desired echo signals are detected and amplified by circuit 16 afterbeing given a predetermined time by a second group of delay lines 15. Again control circuit 17 controls the gain of the amplifying anddetecting circuit 16 and a brightness modulation circuit 18 converts theecho signals supplied from the amplifying and detecting circuit 16 tobrightness signals for displaying the echo image.

A blanking control circuit 19 actuates blanking signals to thebrightness signal output from the brightness modulation circuit 18 forcomposing the echo signals, as determined previously, to providedisplay. A beam direction sweeping circuit 20 generates sweeping wavesrelating to the depth direction, or the acoustic beam direction in caseof the echo image display, and a scanning line position signal generator21 generates the scanning position designating signals corresponding tothe beam positions in case of the echo image display. A displayapparatus 22 displays the echo images by utilizing each output from thebrightness modulation circuit 18, the beam direction sweeping circuit 20and the scanning line position signal generator 21.

A control apparatus 23 controls each operation of the group of pulsers12, the groups of delay lines 13 and 15, the pulse oscillator 14, theamplifying and detecting circuit 16, the gain control circuit 17, theblanking control circuit 19, the beam direction sweeping circuit 20 andthe scanning line position signal generator 21, as described previously,to govern the operation of the whole system.

The control apparatus 23, as shown in FIG. 6, comprises a basic clockgenerator 31, a control pulse generator 32, a delay line control circuit35 and a gain characteristic exchanger 36. The control pulse generator32, comprising timers, frequency dividers and the like, generatessquare-waves, saw-tooth waves and step waves which are provided with avariety of duty periods according to a clock pulse generated from theclock generator 31. The delay line control circuit 33 selects an analogswitch included in the group of delay lines 13 for selecting the valueof delay time to perform a scan and focus of acoustic wave beams. Thepulser selecting circuit 34 selects the pulser simultaneously actuatedof the group of pulsers corresponding to each transducer element of theprobe 11. The delay line control circuit 35 operates substantiallysimilar to the delay line control circuit 33 about the group of delaylines 15. The gain characteristic exchanger 36 controls the gaincharacteristic for every rate pulse.

In operation, trigger pulses are generated by the pulse oscillator 14 inresponse to pulses of the control apparatus 23 to actuate the group ofpulsers 12 through the group of delay lines 13. The unit transducers ofthe probe 11 are oscillated and acoustic pulses are transmitted from theprobe 11.

The acoustic pulses are transmitted out from the probe 11 into thesubject and are reflected from a surface in the subject which differs inacoustic impedance, and the subject reflected pulses, namely the echopulses of the transmission pulses, are returned to the probe 11 to bereceived by the unit transducers and to be supplied to the group ofdelay lines 15 as inputs. The echo wave signals received are applied tothe group of delay lines 15 and are amplified and detected by thecircuit 16 after the predetermined time required to generate echosignals.

Then, the gains of echo signals from the deep portion and the shallowportion in the subject are controlled by the gain control circuit 17 asexplained in more detail hereinafter. The echo signals generated by theamplifying and detecting circuit 16 are converted to brightness signalsby the brightness modulation circuit 18 and supplied to the displayapparatus 22. The display of the brightness output signals from thebrightness modulation circuit 18 is controlled by the blanking controlcircuit 19. In order to display a tomogram of the echo image, beamscanning signals are supplied from the beam direction sweeping circuit20 and the scanning line position signal generator 21.

The timing correlation of the various operations of the circuitarrangement shown in FIG. 5 is indicated in the timing charts of FIG. 7.

FIG. 7(a) shows a wave form of the rate pulses supplied to the probe 11.The rate pulses are constructed as a pulse train which comprises pulsesRP1a, RP1b, RP2a, RP2b, . . . having a time interval T1. A wave form ofthe received echoes corresponding to the rate pulses is shown in FIG.7(b). In this case, for example, when the rate pulse RP1a is generated,the acoustic wave pulses are emitted by the group of unit transducers(for example, which comprises eight unit transducers) having theaperture S1 within the transducer array TL shown in FIG. 8. The beamsare focused by the known controlling method of group of delay lines 13,15 at transmission and reception, respectively, to define the acousticbeam focus at the comparatively short distance R_(n) from the transducerarray TL. The effective acoustic beams in case of the transmission andthe reception corresponding to the rate pulse RP1a are shown with SB₁ inFIG. 8.

When the rate pulse RP1b is generated, the acoustic pulses are emittedat same beam position to the one in case of the rate pulse RP1a, i.e.the center of the effective acoustic beam SB1 is identical to that ofthe beam SB2, by a group of unit transducers which have the aperture S₂(which is more than S₁ and comprises, for example, twelve unittransducers) within the transducer array TL shown in FIG. 8. The beamsare focused to put the beam focusing distance on R_(f), (R_(f) >R_(n)).The effective acoustic beams owing to the rate pulse RP1b are shown withSB₂ in FIG. 8.

Next, when the rate pulse RP2a is generated, the aperture is S₁ and thefocusing distance is Rn at the beam position shifted one step, i.e. onelateral width of the unit transducer, from case of the rate pulse RP1aand when the rate pulse RP2b is generated, the aperture is S₂ and thefocusing distance is R_(f), then in turn repeating the same operation asjust described.

Thus, the display of the tomographic echo images is performed asfollows:

FIG. 7(c) shows a wave form of the sweeping wave signals generated fromthe beam direction sweeping circuit 20 and the sweeping wave signals aresynchronized to the rate pulses as shown. FIG. 7(d) shows a wave form ofthe stepping wave signal generated from the scanning line positionsignal generator 21 for designating the scanning line position and inthis case the signal shifts by predetermined levels every two ratepulses corresponding to the change of beam position.

FIG. 7(e) shows a wave form of the blanking signal generated from theblanking control circuit 19 for composing the echo signals. When thefocusing distance is R_(n) (namely, the short distance), the blankingsignals operate as unblanking, only receiving at a period of time TO ofthe echo from the short distance acoustic field to display the echo dataat the short distance. When however, the focusing distance is R_(f)(namely, the long distance), the blanking signals operate as unblankingonly receiving at a period of time (TI-TO) to display the echo data atthe long distance. In this case, the short distance echo receiving timeTo is determined by the following formula on the basis of the distanceR_(o) from the transducer array to the intersecting point focusedrespectively on the short and long distance:

    TO=Ro/C

where: C is the acoustic wave velocity in the subject. Although theshort distance beam SB1 and long distance beam SB2 are each blanked whenthe other is showing, since they remain on the same scanning line, asimple continuous scanning line is visible on the display to the eye ofthe observer. Thus, the echo signals on the basis of the acoustic wavebeam transmission and reception corresponding to two rate pulses and twolengths of acoustic wave beams are composed into a single scanning lineon the display appearatus.

Also, FIG. 7(f) shows a wave form of the gain control signals suppliedto the amplifying and detecting circuit 16 from the gain control circuit17. The gain control signals have respectively different values aboutthe two lengths of acoustic wave beams which respectively differ with anincline of the sensitivity-time control (S.T.C.) corresponding to thedepth so as to improve quality of the display in which the sensitivitybecomes discontinuity at the intersecting point (the position ofdistance R_(o)) between the two lengths of acoustic beams owing to thediscord in the difference in intensity in the two lengths of acousticwave.

As a result, the images displayed on the display apparatus 22, as shownin FIG. 9 are a composed image of the short distance acoustic field(time TO) reflecting a good short distance characteristic due to theshort distance focus and the small aperture, and the long distanceacoustic field (the period of time TI-TO) reflecting a good longdistance characteristic due to the long distance focus and the largeaperture. Thus, good lateral resolution for the short distance and thelong distance are shown extending the whole imaging distance on thedisplay. Although the blanking time exists in the process of composingeach scanning line, there is no visual distinction of discontinuity andthere is no observation problem in practice even though a time delay forimage-defining actually exists. Since the composing is performed witheach scanning line, there is no decrease of the scanning line densityand there is no observed discontinuity.

Moreover, the invention can be carried out in a variety of modificationsin the scope of the invention without limiting it to the embodiment asdescribed. Although, it has been found that a change in focus toaccommodate an increase in depth is best achieved by increasing thenumber of ultrasonic transducers to be driven, it is known that thefocus distance may be changed while using the same number of actuatedtransducers and, indeed, the focus distance may be changed with thenumber of transducers inversely related to the depth. Moreover,ultrasonic imaging apparatus may be constructed wherein the number oftransducers is changed with a fixed focus distance. Further, showing thetiming charts corresponding to ones in FIG. 7(a) through (f) in FIG.10(a) through (f), the period of time (TI-TO) within ones correspondingto the rate pulses RP_(1a), RP_(2a), . . . may be eliminated.

As shown in FIG. 10, the rate pulses RP1a, RP2a, . . . are emitted for aperiod TO only, and the rate pulses R, P1b, RP2b . . . , having thelonger focal distance, are emitted for a longer period of time TI. Asfurther shown by FIG. 10(e) the beams RP1a, RP2a, . . . with the shorterfocusing distance are unblanked, and the beams RP1b, RP2b, . . . areblanked during the period TO but are unblanked during the period TI-TO.

Furthermore, three or more acoustic beams may be composed for forming ascanning line according to the principles of the invention.

Moreover, the invention may apply not only to linear scan but also toother scan systems such as sector scan, radial scan and the like.

As described above, in accordance with the invention, ultrasonic imagingapparatus can be provided wherein good lateral resolution can beobtained without decrease of scanning line density and withoutobservable discontinuity of the scanning lines.

What we claim is:
 1. An ultrasonic imaging apparatus for displaying atomographic image of a subject to be examined comprising:means foremitting directed acoustic wave beams of various focusing distances intoa subject; beam control means coupled to said emitting means fordirecting at least a series of acoustic wave beams, each seriesincluding at least two consecutive wave beams having a common axis butsubstantially different respective focusing distances from said emittingmeans; means for receiving the reflected wave beams corresponding toeach of the consecutive wave beams of a series; receiver control meanscoupled with said receiving means for controlling the duration of timethe receiver means receives the reflected wave beams so that theduration of reception of at least one of said consecutive acoustic wavebeams in a series is different; modulating means coupled to saidreceiving means for generating modulated scanning lines corresponding toeach of the received reflected wave beams of a series; display controlmeans coupled with said modulating means for composing the modulatedscanning lines corresponding to each of the consecutive wave beams of aseries into a single image scanning line, said display control meansincluding (i) means for unblanking the portions of each said respectivemodulated scanning line that corresponds to the respective range offocusing distance of each of said consecutive acoustic beams and forblanking the remaining portions, if any, and (ii) means for composingsaid unblanked portions of the consecutive acoustic beams of each seriesinto a single image scanning line; and display means coupled with saiddisplay control means for displaying a tomographic image of the subject.2. The apparatus of claim 1 wherein said emitting means can emitacoustic beams of various apertures and wherein said beam control meanscontrols at least the aperture of the acoustic wave beam.
 3. Theultrasonic imaging apparatus of claim 2 wherein said beam control meanscontrols both the aperture of the acoustic wave beams and the focus ofthe acoustic wave beams.
 4. The ultrasonic imaging apparatus of claim 1wherein said modulating means is a brightness modulation circuit andwherein said display control means includes a blanking control circuit.5. The apparatus of claim 1 wherein the first wave beam of a series hasa shorter focal distance than the second consecutive wave beam of aseries.
 6. The apparatus of claim 5 wherein said receiver control meanscauses the receiving means to receive the reflection of the second beamfor a longer time than the first beam.
 7. The apparatus of claim 5wherein the unblanking means blanks a portion of the modulated scanningline associated with the beam having a longer focal distance but doesnot blank at all the modulated scanning line associated with the beamhaving the shorter focal distance.
 8. The apparatus of claim 1 whereinthe beam control means causes the emitting means to emit the acousticwave beam of a series with a shorter focusing distance for a shorterperiod of time than the corresponding acoustic wave beam of therespective series with a longer focusing distance.
 9. The apparatus ofclaim 1 wherein the receiver control means causes the receiving means toreceive the reflected acoustic beam corresponding to the acoustic beamof a series with the shorter focusing distance for a shorter period oftime than the corresponding reflected acoustic beam of the respectiveseries corresponding to the acoustic beam with the longer focusingdistance.
 10. The apparatus of claim 1 wherein said beam control meansdirects more than one said series of consecutive acoustic wave beams.11. The apparatus of claim 10 wherein said beam control means directs atleast one of said more than one series of acoustic wave beams along adifferent axis.